The invention relates to a large transmission gearwheel which is produced from a plurality of individual components welded together and a method for producing such a large transmission gearwheel.
It is known to produce large transmission gearwheels, i.e. gearwheels with diameters of more than 600 mm, as one-piece gearwheels.
A drawback with the one-piece configuration of large gearwheels is that they are extremely disadvantageous in terms of the material required and production weight. As a solution to the weight problem it is known to design large gearwheels with the incorporation of beads. To this end, material is removed during the course of a turning operation of the lateral surfaces of the wheel. As a result, however, the production costs are increased as further cutting operations are required for incorporating the beads and the removed material results in costs in the purchase of unmachined parts.
A further problem occurs with case-hardening. During this process, a large amount of energy is introduced into the component which, during the quenching process, may lead to significant component deformation. This component deformation has to be compensated both in advance by costly structural measures and retrospectively by a corresponding removal of material. As the retrospective removal of material has to take place in the hardened state of the component, it is associated with significant costs and accordingly is not cost-effective. As a structural countermeasure, it is firstly known to design the component with corresponding allowances so that it is possible to remove material retrospectively to the desired dimensions. Secondly, for improved hardenability and/or for improving the quenching process, boreholes are incorporated in the component which are intended to ensure that the quenching medium is flushed around the component more uniformly, in order to achieve an improved temperature profile of the component during the cooling thereof and thus reduced deformation.
In spite of these expensive and complex countermeasures, however, in practice it is only possible with difficulty to restrict the component deformation to limited regions and in a controlled manner. As a result, large gearwheels, which are configured in one piece and which are produced in the manner described above, are a cost-efficient compromise between material costs and production costs but as a whole only cost-effective to a limited degree.
As an alternative to one-piece gearwheels, hybrid gearwheels which are made up of a plurality of components are known, and namely consisting of at least one hub, a wheel body arranged on the hub and a toothed ring arranged on the outer periphery of the wheel body. Very different embodiments of such hybrid large transmission gearwheels which essentially differ in the manner in which the individual components are fastened together and in the design of the individual components are already known in the prior art. The textbooks: Machine Elements by Roloff Matek and Machine Elements by Niemann Winter Höhn, and/or patent documents, such as DE 911 500 B, DE 917 589 B and DE 82 05 946 U1, provide an overview of the prior art for the production of hybrid large transmission gearwheels.
The fastening of individual components to one another may be carried out mechanically. Thus the toothed ring may be screwed, for example, to the wheel body. Alternatively, it is also known to shrink the toothed ring onto the wheel body, wherein the toothed ring is frequently secured against a relative movement with regard to the wheel body by corresponding positive locking devices.
Alternatively, the individual components may also be connected together by a material connection, by means of welding, for which nowadays welding methods using consumable electrodes are exclusively used. In addition to the reliability of the gear teeth, welded gearwheels have to be designed relative to their interface properties. The joint interfaces are made up from the combinations of components including the toothed ring and wheel body (so-called gear rim), the wheel body and hub as well as the hub and shaft. These joints have to be included in the design calculations and production planning. Moreover, the cost thereof in terms of production technology and the subsequent effect thereof on component function also has to be taken into account.
The wheel body may be configured in one piece, for example as a cast part. It is also possible to manufacture the wheel body as a welded structure. Welded wheel bodies are used, in particular, in the construction of large transmissions in quantities of 1 to 5, as they often represent the only possibility of economical production.
A known production sequence for hybrid large transmission gearwheels is, for example, the provision of unmachined parts, pre-turning, welding, gear teeth cutting, optionally induction hardening, gear teeth grinding and subsequent quality control.
The welding work carried out in hybrid large transmission gearwheels is exclusively implemented by means of MIG welding (metal inert gas welding), electrode welding or submerged arc welding. All these methods use consumable electrodes and a relatively high amount of energy per unit distance and therefore have a significant effect on the metallurgical properties of the connection and the basic material. The welding process as such causes high residual stresses in the steel structure. Therefore, it is usual nowadays for the person skilled in the art to carry out stress relief tempering after the welding treatment in order to dissipate stresses, so as to prevent further production steps potentially releasing these stresses as deformation, such as for example when incorporating beads, when asymmetrically reducing the wall thickness or the like. These deformations which accompany the production process are known and lead to faults in the welded structure. Accordingly, highly accurate dimensional machining of a welded large transmission gearwheel is only possible after carrying out stress relief tempering.
The hardening of the welded component takes place by means of induction hardening. This method has the advantage that only a relatively small amount of thermal energy is introduced locally into the component, whereby a high level of component deformation is prevented.
Case-hardening of the welded component, as is used in large transmission gearwheels configured in one piece, is not used in hybrid large transmission gearwheels due to the high residual stresses introduced by the welding process using consumable electrodes. In large transmission gearwheels a relatively low level of component deformation already leads to considerably greater deviations in shape than in small gearwheels, due to the relatively large dimensions, in particular the large diameter of the gearwheel. The deformation due to the dissipation of the residual stress as a result of welding and the deformation due to hardening would cumulatively be so great that costly mechanical post-treatment might be necessary in the hardened state of the component. However, this is undesirable as it leads to substantial costs. Accordingly, in known large transmission gearwheels the toothed ring is always manufactured from hardened and tempered steel.
A drawback with hybrid large transmission gearwheels available nowadays, relative to large transmission gearwheels configured in one piece, is that they are only able to be used for small surface loads as the hardened and tempered toothed rings have a lower load-bearing capacity than case-hardened toothed rings. Whilst the flank strength in hybrid large transmission gearwheels ranges from approximately 600-800 N/mm2, large transmission gearwheels which are configured in one piece and case-hardened have a flank strength of approximately 1500 N/mm2, and with carbonitriding up to 1700 N/mm2. A further drawback is that the aforementioned welding methods are highly manual, which is why they are only able to be used cost-efficiently in batch production. The detection of process data during mass production does not currently form part of the prior art and is only able to be documented by means of retrospective quality controls.
Due to the dimensions in small gearwheels, deviations arising from component deformation lead to fewer deviations of shape than in large transmission gearwheels which is why in hybrid large transmission gearwheels the aforementioned problem of deformation introduced into the component by welding and hardening only plays a minor role, in particular if only lower grade materials are required for the gear teeth as they are used in the context of fatigue strength. This is the case in mobile transmission applications, such as for example in heavy goods vehicles and utility vehicles.